1. Field of the Invention
The present invention relates to a radio packet communication system and a radio packet communication method for performing radio packet communication between a base station and a mobile station. The present invention also relates to a base station and a mobile station which are used in the system and the method.
2. Description of the Related Art
Conventionally, a radio packet communication system has been known in which each of base and mobile stations estimates a condition of a propagation path in a transmission frequency toward itself by using a signal received by itself, and determines a transmission-modulation mode for each other.
Specifically, each of the base and mobile stations estimates a condition of a propagation path in a transmission frequency toward itself by use of a received signal included in a time slot in a transmission frame, and then determines the transmission-modulation mode which can transmit the largest quantity of information within a given transmission error rate under the estimated condition of the transmission path. Subsequently, each of the base and mobile stations transmits data including information indicating the determined transmission-modulation mode by use of a time slot in a subsequent transmission frame. Then, each of the base and mobile stations, which has received the transmitted data, transmits a subsequent transmission frame by the transmission-modulation mode included in the data.
In this radio packet communication system, the base station or the mobile station transmits the transmission frames with the maximum transmission power by use of the transmission-modulation mode which can transmit the largest quantity of information within the given transmission error rate under the estimated condition of each transmission path. Therefore, the base station or the mobile station can achieve the maximum communication speed. That is, even when the condition of the propagation path changes, the base station or the mobile station modifies the transmission-modulation mode, but does not modify the transmission power.
A basic concept of the radio packet communication system is to reduce the number of packets to be transmitted in the same time slot or the number of mobile stations transmitting packets in the same time slot, so as to enable each packet or each mobile station to use many radio resources instantaneously, to complete transmission as soon as possible, and to offer the used radio resources to a subsequent packet or a different mobile station.
Moreover, there has been conventionally known a technique of adaptively changing a digital modulation mode in a radio packet communication system using a CDMA method, in which each of base and mobile stations spreads and transmits a digitally-modulated modulation signal to each other, and in which a downlink transmission frequency is different from an uplink transmission frequency.
Specifically, each of the base and mobile stations detects a signal to interference power ratio (SIR) which is an interference amount in a received signal included in a transmission frame, then selects a digital modulation mode out of a plurality of digital modulation modes which can transmit the largest quantity of information within a given transmission error rate based on the detected interference amount, and then transmits data including information indicating the selected digital modulation mode to each other. In this way, the base station or the mobile station can change the digital modulation mode adaptively.
This radio packet communication system tracks a variation in the interference amount attributable to a traffic variation, phasing, or the like, by detecting the interference amount at the base station or the mobile station, and uses the digital modulation mode which can transmit the largest quantity of information in accordance with the interference at that moment. In this way, the radio packet communication system can use the frequency effectively.
However, in the above-described radio packet communication system, when many mobile stations are located close to a cell border, the interference amount with an adjacent cell fluctuates instantaneously and significantly because of an instantaneous increase in the traffic in the cell. Accordingly, there has been a problem in that the radio packet communication system has a risk of an increase in occurrence of a communication error in the adjacent cell.
Moreover, in the above-described radio packet communication system, even when the total traffic in the cell is unchanged, the interference amount with the adjacent cell fluctuates instantaneously and significantly because of a variation in distance between the base station and the mobile station. Accordingly, there has been a problem in that a possibility of occurrence of a communication error in the adjacent cell increases.